This invention relates to a spectral radiometer designed to be left in the field on a stand-alone basis for prolonged periods of time (months to years) to measure the spectral characteristics of various earth surface targets in two bands.
For several decades, spectral radiance data have been used to help map and monitor the earth""s surface. The advent of civilian satellite imaging systems in the early 1970s propelled this technology into widespread use both in image format and in situ field measurements. Digital imaging systems carried on-board earth-orbiting satellites collect images using optical systems that record the earth surface spectral characteristics in various bands (e.g., the brightness/color in visible and near-infrared (NIR) spectral bands). Current satellite imaging systems have expanded spectral band coverage compared to those used in the 1970s and early 80s (e.g., short-wave IRxe2x80x94SWIR), resulting in the need for more sophisticated field instruments with increased spectral measurement capabilities. In order to help identify the design of future satellite imaging sensors and to collect spectral radiance data in the field for use with current state-of-the-art satellite and airborne imaging systems, the need to design and build more complex and sophisticated spectral radiometers for in-the-field use has driven both the cost and constraints of such an instrument quite high. The cost of spectral radiometers now range from $12K to $80K for field instruments that can collect data up to 1024 spectral bands. For applications that require only a few bands (e.g., 2 to 6), with a high temporal resolution over a prolonged period of time, using a current state-of-the-art spectral radiometer is out of the question. Both in cost and design the currently available spectral radiometers are not made for long-term stand-alone field use. Therefore, they are used only for short-term studies and applications while the user is in the field, meaning that they cannot provide the high temporal resolution needed for long-term studies and operational monitoring of the earth""s surface.
Spectral radiometers were developed for in-the-field use for remote sensing in the early 1970s. Radiometers with four broad spectral bands were designed first, then the number of spectral bands began to increase and the band width to decrease to allow better total spectral measurements to be collected of the cover types of interest (i.e., soils, vegetation, or water). In the late 1980s and early 1990s, spectral radiometers having over 250 narrow spectral bands were designed and built and now ones with 512 to 1024 bands (hyper-spectral) are becoming the standard. However, there are a number of problems with the use of current hyper-spectral radiometers, including:
1) The amount of data they are designed to collect is typically an xe2x80x9coverkillxe2x80x9d for many applications and operational monitoring of the earth""s surface (e.g., surface waters and general vegetation cover). That is, over 500 bands of spectral radiance data are not needed for many applications, especially for non-research operational and monitoring uses.
2) The cost for the much more complex and sophisticated instruments now on the market ranges from $12,000 to $80,000. This cost is quite high for most non-research applications, especially if several to tens of them are needed for good spatial monitoring over a regional area.
3) The spectral radiometers currently on the market are designed to be used in the field for a relatively short amount of time by a person while doing fieldwork. Current radiometers are not designed to be left in the field for long periods of time (i.e, months to years) in a stand-alone node to automatically collect spectral radiance data with a high temporal resolution.
4) The current spectral radiometers with over 500 bands have narrow bandwidth, so the overall signal-to-noise ratio is low compared to the more broadband width radiometers. Therefore, for low radiance targets, such as surface waters, the noise levels will typically be higher than for those collected by a less complex broadband radiometer.
5) A spectral radiometer with four bands was developed in the early 1970s by Exotech, Inc., for use in the field while the operator was present; it is not designed to be left in the field on a stand-alone basis for prolong periods of time.
Satellite image data can be used to monitor various features on the earth""s surface with additional spectral windows. However, major problems with using satellite image data to monitor surface water parameters, and on-land vegetation cover, are that the combination of temporal and spatial resolutions often needed are well beyond the capability of current satellite imaging systems. To obtain the temporal resolution needed of minutes to hours, and a spatial resolution of one to three meters required to see the surface waters of rivers, or for daily monitoring of vegetation within a small area, a field based instrument is required.
A number of other types of radiometers have been disclosed which have been used for a variety of purposes.
Goetz et al., in U.S. Pat. No. 4,345,840, disclose a hand held, self-contained dual beam rationing radiometer for identifying selected materials that reflect radiation within a predetermined band, preferably in the IR or visible range. The apparatus includes two pivoting optical trains directed toward the same target. Each train has a separate filter for selection of the narrow spectral bands to be ratioed, by means of a dividing circuit, to identify a particular substance based on its known spectral characteristics.
Spiering et al., in U.S. Pat. No. 6,020,587, disclose a device for measuring plant chlorophyll content by collecting light reflected from a target plant. A beam splitter separates the light into distinct wavelength bands or channels. Photo-detectors and amplifiers within the device then process the bands, converting them into electrical signals.
Gupta discloses, in U.S. Pat. No. 4,996,430, a device for distinguishing target objects having substantially identical reflectance ratios for two separated wavelengths (lambda-1, lambda-2), from background objects possessing different reflectance values for the same two wavelengths. This device includes an active optical sensor with first and second transmitters that transmit signals at wavelengths of lambda-1 and lambda-2, respectively. When the transmitted signals reflect off of an object, a receiver senses the reflected signals, which are then processed through a high-speed preamplifier and amplifier, producing voltages V1 and V2 at the receiver""s output. A conventional ratio calculating circuit then calculates a ratio of V1 and V2, which is then compared to a predetermined threshold value.
Levin et al., in U.S. Pat. No. 6,031,233, disclose a handheld spectrometer for identifying samples based upon their IR reflectance. The device includes a window and adjacent optical bench. The optics, which align directly with the sample under investigation, consist of a broad band IR light shining onto an acousto-optic tunable filter crystal, the latter passing narrow band IR light with a swept frequency; a lens focusing the IR light through the window onto the sample; and a reflectance detector aligned with the housing window to detect light reflected from the sample. A computer mounted in the housing compares the reflectance spectrum with stored data and identifies the sample material.
Novison, in U.S. Pat. No. 5,527,062, discloses a portable IR spectrophotometer for testing samples of organic construction materials. A single source of IR light is divided into two beams. The first beam is directed to the surface of the target sample, from which the light is reflected at least four times. The second reference beam is directed toward a neutral surface. The two beams are then combined and focused onto a detector element. The detector output is proportional to the energy absorbed by the test sample. A pen recorder attached to the apparatus generates a graphic xe2x80x9cfingerprintxe2x80x9d of the sample""s reflectance spectrum.
Typically, spectral bands used to study and monitor water parameters fall within the blue and green spectral bands, not red and near IR as to be used in this application.
Traditionally, spectral radiance measurements must be converted to surface reflectance, requiring a more complicated setup and additional instrument capabilities in the form of a calibrator or a solar irradiance measurement. By ratioing the spectral radiances of two bands, as to be done in this design, the need to collect measurements of either a calibrator or solar irradiance at the same time the target surface readings are collected is not needed.
A main objective of the present invention is to overcome the deficiencies in the existing instruments and to provide a robust, dual-band spectral radiometer suited for long-term stand-alone field use. By using the ratio of spectral radiance values to correlate to the parameters of interest, rather than spectral reflectance values as is typically used, it eliminates the need for either calibrator or solar irradiance measurements, thereby reducing the cost and complication encountered when these extra measurements are needed. Two immediate applications of this design will be to measure and monitor the spectral characteristics of water surfaces, which can be correlated to water parameters such as total suspended sediment concentration (total SSC) and turbidity, and on-land vegetation spectral signatures, which can be correlated to vegetation cover/density, with possible correlation to vegetation health/water stress.
This invention includes a method to effect water surface and on-land vegetation long-term stand-alone monitoring by utilizing the same two spectral bands. For these applications the two spectral bands have a broadband width and cover the visible red and near-IR portions of the spectrum. However, the spectral radiometer""s general design is such that for other applications that will arise a different set/pair of two bands can be used (e.g., blue and red, and/or narrow instead of broad band widths). Also, if it is determined that some applications required more than two bands (e.g., five or six), then sets of this radiometer can be used (2 bands per radiometer times 3 radiometers will allow a six band set up, if needed).
Again, another main objective of the invention or method is to provide a means to correlate spectral data collected in the field with this instrument to surface parameters of interest, such as total SSC or vegetation density, without the need to also collect spectral measurements of a calibrator or solar irradiance at the same time.
This invention includes a new procedure that uses the ratio of measured spectral radiances in two bands. In the case of surface waters and on-land vegetation it uses broad bands in the visible red and near-IR portions of the spectrum, for long-term operational monitoring of the parameters of interest. The ratio of the spectral radiances in two bands has a direct and high correlation to the ratio of reflectance values, therefore, this can often serve as a replacement to using actual surface reflectance in the correlation of the spectral measurements to the surface parameter of interest. This means the need for either a calibrator or solar irradiance measurements to be collected at the same time is eliminated, thus making the procedure both less expensive and less complex than procedures that use reflectance values. It should be noted that if either a calibrator or solar irradiance measurements are needed, the maintenance requirements of a long-term stand-alone set up will dramatically increase. This will be because the calibrator or the sky looking optics will need to be kept clean so the measurements are reliable and comparable from day-to-day.
The instrument in the present invention is designed to collect spectral radiance measurements in two bands. For the surface waters and on-land vegetation monitoring applications it will use broad-width bands in the visible red and near-infrared portions of the spectrum. The spectral radiance values from these two broad-width bands have a good correlation with several surface water parameters, including total suspended sediment concentration (total SSC) related to silt and clay as well as turbidity, on both inland and coastal waters. The spectral radiance values from these same two spectral bands also have a good correlation with vegetation cover or density, and have been shown to often correlate with vegetation maturity and, in some cases, vegetation healthxe2x80x94water stress.
Two immediate applications of the instrument are to measure the spectral characteristics of water surfaces or on-land vegetation to map and monitor the total suspended sediment concentrations and turbidity of water or vegetation cover/density.